1. Field of the Invention
The present invention relates to a hologram recording method and a hologram recording apparatus. The invention particularly relates to a method and an apparatus that record digital data as a hologram on an optical recording medium.
2. Description of the Related Art
In holographic data storage, binary digital data “0, 1” are converted into a digital image (signal light) as “bright, dark”, and recorded and reproduced as a hologram. The signal light is Fourier-transformed by a lens, and a Fourier transformation image is irradiated onto an optical recording medium. Since the Fourier transformation image has infinite spread at its focal plane in principle, a recording region spreads. For this reason, in the holographic data storage, there is a problem that high-density recording cannot be achieved.
In order to solve this problem, the inventors of this invention propose a method of shielding a specific component of a Fourier transformation image of signal light (Japanese Patent Application Laid-Open (JP-A) No. 2000-66565). The Fourier transformation image of a digital image is a regular diffraction grating by means of Fraunhofer's diffraction, and has 0-th to n-th order components. The orders here are the order of bright spots which appear at every distance from the 0-th order (center) to ζ=fλ/d on a Fourier transformation plane, and are determined by a focal distance f of a lens, a recording wavelength λ, and a pixel pitch d of a spatial light modulator.
In the method described in JP-A No. 2000-66565, a light blocking body is disposed between a lens and an optical recording medium, and an aperture is formed in the light blocking body so that the 0-th order component of signal light is shielded and only an component of a specific order is transmitted, and an image edge portion of the signal light is recorded as a hologram. JP-A No. 2000-66565 concretely discloses that the 1-st order to 3-rd order components of the signal light are recorded. According to this method, useless exposure due to the 0-th order component is suppressed, and a recording region of each data page can be small.
Each high-order component which is the 1-st order and more, however, includes a DC component of the signal light. The DC component of the signal light is an unnecessary component for recording and reproducing of the image edge portion of a digital image. In the method described in JP-A No. 2000-66565, therefore, even if the 0-th order component is shielded, the useless exposure by means of the DC component of each high-order component cannot be prevented. As a result, there is a problem that S/N (signal-noise ratio) is reduced, and a dynamic range of an optical recording medium is wasted.
In general, the light intensity of the DC component is higher by 2 digits than a high-frequency component. As a potential of the optical recording medium, a performance of 1 terabyte/disc or more is demonstrated, but when digital data are recorded as a hologram, the demonstration level is several dozens gigabytes/disc. It is considered that this gap is generated because most of the dynamic range of the optical recording medium is wasted by the exposure of the DC component.
Further miniaturization of the recording region is also essential for high-density recording.